1. Field of the Invention
The present invention relates to a screen measurement system, and more particularly, to a resistive touch screen measurement system.
2. Description of the Prior Art
Primary resistive screen measurement systems in use recently comprise single-ended mode resistive screen measurement systems and differential mode resistive screen measurement systems.
Please refer to FIG. 1, which is a diagram of an equivalent circuit of a resistive touch screen measurement system utilizing the differential mode. Please also refer to FIG. 2, which is a status table of the elements of the resistive touch screen measurement system 100 measuring a first coordinate factor and a second coordinate factor for determining a coordinate of a touch point corresponding to a touch screen. The coordinate is a two-dimensional coordinate utilized for representing a planar position. Therefore, the first coordinate factor represents the X-coordinate of the coordinate, and the second coordinate factor represents the Y-coordinate of the coordinate. The resistive touch screen system 100 comprises a touch-sensing system 101, an analog-to-digital converter 103, and a touch screen 120 disposed on the touch-sensing system 101 for receiving a signal generated from an artificial touch and transmitting the signal to the touch-sensing system 101. The touch screen 120 is not included in the touch-sensing system 101 and overlaps with the touch-sensing system 101. The resistive touch screen measurement system 100 further comprises a first voltage source 151 and a second voltage source 153 for providing a bias voltage of the touch-sensing system 101. The touch sensing system 101 comprises a first transistor 105, a second transistor 107, a third transistor 109, a fourth transistor 111, a first resistor 113, a second resistor 115, a third resistor 117, a fourth resistor 119, and a touch point 129. The first resistor 113 is coupled to the first transistor 105 at a first output 121. The second resistor 115 is coupled to the second transistor 107 at a second output 123. The third resistor 117 is coupled to the third transistor 109 at a third output 125. The fourth resistor 119 is coupled to the fourth transistor 111 at a fourth output 127. The touch point 129 is coupled to the first resistor 113, the second resistor 115, the third resistor 117, and the fourth resistor 119. The analog-to-digital converter 103 comprises an analog input 131, a reference upper bound input 133, a reference lower bound input 135, and a digital output 137. The analog input 131 is coupled to the first output 121 through a switch 171 and is coupled to the third output 125 through a switch 173. The reference upper bound input 133 is coupled to the first output 121 through a switch 175 and is coupled to the third output 125 through a switch 177. The reference lower bound 135 is coupled to the second output 123 through a switch 179 and is coupled to the fourth output 127 through a switch 181. The first resistor 113 and the second resistor 115 are physically the same resistor, and the third resistor 117 and the fourth resistor 119 are physically the same resistor. However, the artificial touch on the touch screen 120 generates a corresponding signal at the touch point 129. Therefore, the artificial touch temporarily separates the first resistor 113 and the second resistor 115. The artificial touch also temporarily separates the third resistor 117 and the fourth resistor 119.
While measuring the first coordinate factor of the coordinate of the touch point 129 on the touch screen 120, the third transistor 109 and the fourth transistor 111 are both turned on, and the first transistor 105 and the second transistor 107 are both turned off. A first output voltage is generated at the touch point 129, and a second output voltage is generated at the first output 121 from the first output voltage through the first resistor 113. At this time, the switch 171 coupled to the analog input 131 and the first output 121 is turned on, therefore, the second output voltage is inputted to the analog-to-digital converter 103 through the analog input 131. Moreover, the switch 177 coupled to the reference upper bound input 133 and the third output 125 is turned on, and the switch 175 coupled to the reference upper bound input 133 and the first output 121 is turned off. Therefore, the voltage at the third output 125 is inputted to the analog-to-digital converter 103 through the reference upper bound input 133 and is regarded as an upper bound reference voltage. At this time, the switch 181 coupled to the reference lower bound input 135 and the fourth output 127 is turned on, and the switch 179 coupled to the reference lower bound input 135 and the second output 123 is turned off. Therefore, the voltage at the fourth input 127 is inputted to the analog-to-digital converter 103 through the reference lower bound input 135. Through the functions of the analog-to-digital converter 103, the inputted second output voltage is transformed into a corresponding digital signal for representing the first coordinate factor of the coordinate and outputting the first coordinate factor through the digital output 137.
While measuring the second coordinate factor of the coordinate of the touch point 129 on the touch screen 120, the first transistor 105 and the second transistor 107 are turned on, and the third transistor 109 and the fourth transistor 111 are turned off. Therefore, a third output voltage is generated at the touch point 129, and a fourth output voltage is generated from the third output voltage at the third output 125 through the third resistor 117. At this time, the switch 173 coupled to the analog input 131 and the third output 125 is turned on, and the switch 171 coupled to the analog input 131 and the first output 121 is turned off. Therefore, the fourth output voltage is inputted to the analog-to-digital converter 103 through the analog input 131. Moreover, the switch 177 coupled to the reference upper bound input 133 and the third output 125 is turned off, and the switch 175 coupled to the reference upper bound input 133 and the first output 121 is turned on. Therefore, the voltage at the first output 121 is inputted to the analog-to-digital converter 103 through the reference upper bound input 133 and is regarded as an upper bound reference voltage. At this time, the switch 181 coupled to the reference lower bound input 135 and the fourth output 127 is turned off, and the switch 179 coupled to the reference lower bound input 135 and the second output 123 is turned on. Therefore, the voltage at the second output 123 is inputted to the analog-to-digital converter 103 through the reference lower bound input 135 and is regarded as a lower bound reference voltage. Through the functions of the analog-to-digital converter 103, the inputted fourth output voltage is transformed into a corresponding digital signal for representing the second coordinate factor of the coordinate and outputting the second coordinate factor through the digital output 137.
However, while measuring the first coordinate factor of the coordinate of the touch point 129 on the touch screen 120 and after the second output voltage is inputted to the analog-to-digital converter 103, the reference upper bound input 133 must be continuously supplied with the voltage input at the third output 125, and the reference lower bound input 135 must be continuously supplied with the voltage input at the fourth output 127, therefore, the third transistor 109 and the fourth transistor 111 cannot be turned off. If the third transistor 109 and the fourth transistor 111 are turned off at this time, the upper bound reference voltage and the lower bound reference voltage cannot be maintained anymore, and larger offsets are thus generated. Since the third transistor 109 and the fourth transistor 111 cannot be turned off, the third transistor 109 and the fourth transistor 111 must work for a longer time, and a large power consumption caused by not turning off the third transistor 109 and the fourth transistor 111 is thus generated. It means that a resistive touch screen measurement system utilizing the differential mode is in company with a large power consumption.
Please refer to FIG. 3, which is a diagram of the resistive touch screen measurement system 200 utilizing a single-ended mode. Please refer to FIG. 4 also. FIG. 4 is a status table of the elements of the resistive touch screen measurement system 200 measuring a first coordinate factor and a second coordinate factor for determining a coordinate of a touch point on a touch screen. The coordinate is a two-dimensional coordinate for representing a planar coordinate as well as in FIG. 1. The first coordinate factor of the coordinate represents the X-coordinate of the coordinate. The second coordinate factor of the coordinate represents the Y-coordinate of the coordinate. The resistive touch screen measurement system 200 comprises a touch-sensing system 201, an analog-to-digital converter 203, and a touch screen 220. The touch screen 220 is disposed above the touch-sensing system 201 for receiving a signal generated from an artificial touch and transmitting the signal to the touch-sensing system 201. In FIG. 3, the touch screen 220 is not comprised by the touch-sensing system 201 and overlaps with the touch-sensing system 201. The resistive touch screen measurement system 200 further comprises a first voltage source 251 and a second voltage source 253 for providing bias voltages to the touch-sensing system 201. The touch-sensing system 201 comprises a first transistor 205, a second transistor 207, a third transistor 209, a fourth transistor 211, a first resistor 213, a second resistor 215, a third resistor 217, a fourth resistor 219, and a touch point 229. The first resistor 213 is coupled to the first transistor 205 through a first output 221. The second resistor 215 is coupled to the second transistor 207 through a second output 223. The third resistor 217 is coupled to the third transistor 209 through a third output 225. The fourth resistor 219 is coupled to the fourth transistor 211 through a fourth output 227. The touch point 229 is coupled to the first resistor 213, the second resistor 215, the third resistor 217, and the fourth resistor 219. The analog-to-digital converter 203 comprises an analog input 231, a reference upper bound input 233, a reference lower bound input 235, and a digital output 237. The analog input 231 is coupled to the first output 221 through a switch 271 and is coupled to the third output 225 through a switch 273. The reference upper bound input 233 is coupled to a direct current (DC) voltage source VDD. The reference lower bound input 235 is coupled to the ground. The first resistor 213 and the second resistor 215 are physically the same resistor. The third resistor 217 and the fourth resistor 219 are physically the same resistor. The artificial touch generated on the touch screen 220 generates a signal on the touch point 229 of the touch-sensing system 201. The artificial touch thus separates the first resistor 213 and the second resistor 215. The artificial touch also separates the third resistor 217 and the fourth resistor 219.
While measuring the first coordinate factor of the coordinate of the touch point 229 on the touch screen 220, the third transistor 209 and the fourth transistor 211 are turned on, and the first transistor 205 and the second transistor 207 are turned off. Therefore, a first output voltage is generated on the touch point 229, and a second output voltage is generated from the first output voltage at the first output 221 through the first resistor 213. At this time, the switch 271 coupled to the analog input 231 and the first output 221 is turned on, and the switch 273 coupled to the analog input 231 and the third output 225 is turned off so that the second output voltage is inputted to the analog-to-digital converter 203 through the analog input 231. Moreover, the DC voltage source VDD at the reference upper bound input 233 is regarded as an upper bound reference voltage, and the ground GND at the reference lower bound input 235 is regarded as a lower bound reference voltage. Through the functions of the analog-to-digital converter 203, the inputted second output voltage coupled is transformed into a corresponding digital signal for representing the first coordinate factor of the coordinate of the touch point and outputting the digital signal from the digital output 237.
While measuring the second coordinate factor of the touch point 229 on the touch screen 220, the first transistor 205 and the second transistor 207 are turned on, and the third transistor 209 and the fourth transistor 211 are turned off. Therefore, a third output voltage is generated at the touch point 229, and a fourth output voltage is generated from the third output voltage at the third output 225 through the third resistor 217. At this time, the switch 273 coupled to the analog input 231 and the third output 225 is turned on, and the switch 271 coupled to the analog input 231 and the first output 221 is turned off. Therefore, the fourth output voltage is inputted to the analog-to-digital converter 203 through the analog input 231. Besides, the DC voltage source VDD at the reference upper bound input 233 is regarded as an upper bound reference voltage, and the ground GND at the reference lower bound input 235 is regarded as a reference lower bound voltage. Through the functions of the analog-to-digital converter 203, the inputted fourth output voltage is transformed into a corresponding digital signal for representing the second coordinate factor of the coordinate of the touch point and outputting the digital signal from the digital output 237.
While measuring the first coordinate factor of the coordinate of the touch point 229 on the touch screen 220 and after the second output voltage is inputted to the analog-to-digital converter 203, the analog-to digital converter 203 does not have to maintain the voltages at the third output 225 and the fourth output 227 in comparison with the analog-to-digital converter 103. Therefore, the power consumption of the resistive touch screen measurement system 200 utilizing the single-ended mode is less than the power consumption of the resistive touch screen measurement system 100 utilizing the differential mode. However, since the upper bound reference voltage of the analog-to-digital converter 203 is the DC voltage source VDD, the lower bound reference voltage of the analog-to-digital converter 203 is the ground GND, the actual voltage at the first output 221 is VDD-Vsd, and the actual voltage at the second output 223 is GND+Vds, therefore, a larger offset is generated in the bias voltage and the gain of the analog-to-digital converter 203. Vsd represents the voltage difference between the source and the drain of the first transistor. Vds represents the voltage difference between the drain and the source of the second transistor 207. The values of Vsd and Vds are both related to bias voltages and temperature of the transistors, and even to the ratio of the width to the length of the transistors. Therefore, the values of Vds and Vsd vary a lot. Moreover, in comparison with the analog-to-digital converter 203, the bias voltage and the gain of the analog-to-digital converter 103 are more precise since the analog-to-digital converter 103 utilizes the voltages outputted at the third output 125 and the fourth output 127 as the upper bound reference voltage and the lower bound reference voltage. While measuring the second coordinate factor of the coordinate of the touch point 229 on the touch screen 220, the same situation takes place.